Exhaust gas emitted from an internal combustion (IC) engine is a heterogeneous mixture that may contain gaseous emissions such as carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NOx) as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust treatment system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
Exhaust gas treatment systems may include selective catalytic reductant (SCR) devices. The SCR device includes a substrate having a washcoat disposed thereon that operates to reduce the amount of NOx in the exhaust gas. The typical exhaust treatment system also includes a reductant delivery system that injects a reductant such as, for example, ammonia (NH3), urea (CO(NH2)2), etc.). The SCR devices make use of the reductant to reduce the NOx. For example, when the proper amount of reductant is supplied to the SCR device under the proper conditions, the reductant reacts with the NOx in the presence of the SCR washcoat to reduce the NOx emissions.
A NOx sensor is typically disposed downstream from the SCR device to sense the level of NOx emissions in the exhaust gas stream. A NOx conversion efficiency of the SCR device can be determined based on the level of NOx that reaches the downstream NOx sensor. If the NOx sensor is performing improperly, however, an incorrect NOx efficiency may be determined such that the performance of the SCR device is misdiagnosed.